The Skinny on Run-flat Tires
By Aaron Robinson
The Black Art of Black Rubber
Tuning tires comes down to forging a series of compromises between conflicting
forces to get the ride and performance balance ideal for your new model. Here
are some examples: stiffer sidewalls create better performance because the
tire deforms less in corners and under acceleration and braking. That creates
the kind of crisp control responses you expect in a sports car. But, stiffer
tires also transmit more bump energy into the chassis, which makes for a
harsher ride. Larger tires have a bigger “contact patch,” the vital link
between the car and Mother Earth. The bigger the contact patch, the more grip
you have and the better the car turns, accelerates, and brakes. But, it also
generates more noise, cuts fuel economy, and has worse sloppy-weather
traction. Softer rubber compounds in the tread provide better grip, but wear
out more quickly.
And so on…
Car engineers are accustomed to juggling these tradeoffs to get the ride and
handling they want. With run-flats, it’s a whole new world. The stiffer
sidewalls of a run-flat, for example, mean smaller contact patches and more
bump-energy transmission. Indeed, the first run-flat tires were cited for
their low grip and harsh ride. As demand for run-flats has increased, the car
industry is becoming smarter about how to adapt the technology.
For starters, the tire makers are devising better rubber formulas that reduce
the ride penalty caused by the reinforced sidewalls. Driving a 2005 Ford
Mustang GMT equipped with Pirelli’s Eufori@ tires through the villages of
southern France turned a lot of heads but didn’t bobble ours over rough
pavement. The automakers have also started tuning their suspensions
specifically for run-flats. One tweak is relaxing the “durometer,” or
stiffness of the suspension bushings, to help cushion impacts that would
better absorbed by a conventional tire.
What Does it All Mean?
If your car didn’t come with run-flats originally, proceed with caution.
Run-flats work best with suspensions that are designed for them from the
outset. You may find the ride too stiff for your liking. If the new car you
are shopping for has run-flats, forget the negative press you have read in the
past. Run-flat technology has rolled a long way since the early days, and it’s
going to continue. Part of Pirelli’s display included a prototype wheel rim
cast with a hollow chamber that is pumped with air. If the tire develops a
leak, a warning signal lights on the dash while a valve allows air to pass
from the chamber to the tire, maintaining tire pressure until you can safely
pull over. Another system in development constantly monitors air pressures,
calling the driver on his or her cell phone if the pressures drop below normal.
Wait. Tires that call you on your cell phone? It’s not just dumb rubber any
Aaron Robinson has spent the past 15 years reporting on cars and the car
industry, first for a national car dealer magazine, then for the weekly trade
newspaper "Automotive News," now as Technical Editor of the world's largest
consumer car monthly, "Car and Driver." When not hunched over a word
processor, he's usually found in his garage trying to save worthless old cars
from a well-deserved trip to the junkyard.